(1) Field of the Invention
The present invention relates to a thermal printhead and a thermal transfer printing apparatus.
(2) Description of the Prior Art
Hitherto, attempts have been made for effecting a gradation printing by thermal transfer printing method by making use of a thermal printhead. It is, however, extremely difficult to change the printing density of each dot formed by each heat generating element. Conventionally, therefore, the gradation control has been made in a false manner by using a plurality of dots in representing a single picture element. This method inconveniently complicates the signal processing and reduces the resolution of the printed image as compared with the density of arrangement of the heat generating elements on the recording head, thus impairing the fineness of the printed picture.
This problem of the prior art will be explained hereinunder with reference to the accompanying drawings.
FIG. 1 schematically shows the construction of a thermal transfer printer. The printer has a thermal printhead 1 having a plurality of heat generating elements 1R arranged in a direction perpendicular to the plane of the sheet of the Figure. A reference numeral 2 designates a thermal transfer ink sheet which is applied to the base film 2a by means of a heat-meltable ink 2b. As shown in the Figure, the thermal transfer ink sheet 2 and a printing paper 3 are adapted to run in the direction of an arrow, through the gap formed between the thermal printhead 1 and a platen 4. In order to ensure a close contact between the printing paper 3 and the thermal transfer ink sheet 2, as well as between the thermal transfer ink sheet 2 and the heat generating elements 1R of the thermal printhead 1, the platen is urged towards the thermal printhead 1.
FIG. 2 shows the detail of the array of the heat generating elements 1a on conventional thermal printhead 1. As will be seen from this Figure, each heat generating element 1R has a heat generating body 1b connected between a pair of electrodes 1a, 1a. In operation, voltages are applied selectively to the heat generating elements 1R in accordance wtih the recording signal, so that the selected heat generating elements 1R generate heat. As a result, the heat-meltable ink 2b on the portions of the thermal transfer ink sheet 2 adjacent the selected heat generating elements is molten and transferred, thus printing the data corresponding to the recording signal.
FIG. 3 shows the relationship between the amount of heat applied to the thermal transfer ink sheet 2 and the density D of the image formed on the printing paper 3.
As will be seen from FIG. 3, the printing density D exhibits an extremely large gradient when the heat amount Q ranges between Q.sub.1 and Q.sub.2, but the curve representing the density D is flat in the region between Q.sub.2 and Q.sub.3.
FIG. 4 shows the states of printing of dots on the printing paper 3 corresponding to the heat amounts Q.sub.1, Q.sub.2 and Q.sub.3, respectively. The printing cannot be made satisfactorily when the amount of heat falls below Q.sub.2, due to failure in the transfer of the heat-meltable ink 2b to the printing paper 3.
Then, as the amount of heat is increased to Q.sub.2, the heat-meltable ink 2b in the whole area corresponding to the heat generating element 1R is molten and transferred to the printing paper 3, thus drastically increasing the printing density. However, when the heat amount is increased beyond Q.sub.2 up to Q.sub.3, the printing density is not increased appreciably and adjacent dots merge into each other undesirably.
Thus, the heat-meltable ink of the thermal transfer ink sheet 2 is not molten sufficiently and, hence, the transfer to the printing paper is not attained satisfactorily, unless the heat applied by the heat generating element exceeds a predetermined level. In other words, the transfer is made or not made depending on whether the heat input exceeds or not a predetermined threshold region which is between Q.sub.1 and Q.sub.2 in the case of the example shown in FIG. 3.
Therefore, it has been difficult to attain a smooth gradation control by linearly changing the heat amount applied to the thermal transfer ink sheet 2, when the printing is conducted on the thermal transfer ink sheet 2 by means of the conventional thermal printhead.
For this reason, when it is necessary to effect a gradation printing by using a thermal printhead having heat generating elements each consisting of a heat generating body 1b connected between a pair of electrodes 1a, 1a as shown in FIG. 2, it has been a common measure to use a matrix of a plurality of heat generating elements for representing each picture element and to change the number of dots, i.e., the heat generating elements, taking part in the heating operation in each matrix, thus attaining a gradation control by controlling the area occupied by dots as shown in FIG. 5.
More specifically, FIG. 5 exemplarily shows the case where each picture element is constituted by four dots, with each square representing the printing area for one dot.
When the gradation printing is conducted in accordance with the explained method by making use of the conventional thermal printhead shown in FIG. 2, the density of the picture elements, i.e., the resolution, is undesirably lowered as compared with the density of arrangement of the heat generating elements on the thermal printhead, so that the fineness of the print is impaired and a complicated procedure is required for processing the signals.